A circuit board in which a metal circuit layer having electrical conductivity is integrally bonded to the surface of a ceramic substrate such as an alumina (Al.sub.2 O.sub.3) sintered body having excellent insulating characteristics with a soldering material, and a semiconductor element is mounted at a predetermined position of the metal circuit layer is popularly used.
A ceramic sintered body containing silicon nitride as a main component has excellent heat resistance in a high-temperature environment of 1000.degree. C. or more, and has excellent thermal-shock resistance. For this reason, as a structural material for high temperature which can be replaced with a conventional heat resistant super alloy, the ceramic sintered body is tried to be applied to various refractory heat-resistant parts such as gas turbine parts, engine parts, or mechanical parts for making steel. In addition, since the ceramic sintered body has high corrosion resistance to a metal, the ceramics sintered body is tried to be applied as a melt-resisting material for a molten metal. Since the ceramic sintered body has high abrasion resistance, the ceramic sintered body is tried to be actually applied to a slidable member such as a bearing or a cutting tool.
As the composition of a conventional silicon nitride ceramic sintered body, the following is known. That is, an oxide of a rare earth element or alkaline earth element such as yttrium oxide (Y.sub.2 O.sub.3), cerium oxide (CeO), or calcium oxide (CaO) is added as a sintering assistant to silicon nitride material powder. By using such a sintering assistant, sintering characteristics are improved, and high density and high strength can be obtained.
A conventional silicon nitride sintered body is manufactured as follows. That is, a silicon nitride source powder is added with the above sintering assistant to be molded. The molded member is sintered in a sintering furnace at a temperature of about 1,600.degree. C. to 2,000.degree. C. for a predetermined time, and then cooled in the furnace. The resultant sintered body is subjected to grinding and polishing processes.
However, in a silicon nitride sintered body manufactured by the above conventional method, although mechanical strength such as a toughness or tenacity value is excellent, thermal conductivity characteristics are considerably degraded in comparison with other sintered bodies such as an aluminum nitride (A1N) sintered body, a beryllium oxide (BeO) sintered body, and a silicon carbide (SiC) sintered body. For this reason, the silicon nitride sintered body is not actually used as a material for electronics such as a semiconductor circuit board whose heat radiation characteristics are required, and the application range of the silicon nitride sintered body is disadvantageously narrow.
Since an aluminum nitride (A1N) sintered body has a high thermal conductivity and a low heat expansion coefficient in comparison with other ceramic sintered bodies, the aluminum nitride sintered body is popularly used as circuit board parts or packaging materials for mounting a semiconductor element (chip) whose high speed, high output, multi-function, and increase in size are developed. However, an A1N sintered body having sufficient mechanical strength cannot be obtained. For this reason, damage occurs in the step of packaging a circuit board, and the packaging step becomes cumbersome, thereby degrading the manufacturing efficiency of a semiconductor device.
More specifically, when a circuit board having a ceramic substrate such as the aluminum nitride sintered body or an aluminum oxide sintered body as a main constituent material is to be fixed to a packaging board with screws or the like in the assembly step, a circuit board is damaged by slight deformation caused by the pressing force of the screws or shock in handling, the production yield of a semiconductor device may considerably decrease.
For this reason, a large circuit board having a large substrate area is not easily formed. In assembling a semiconductor device, a large number of circuit boards are respectively incorporated in the device main body in accordance with the number of required functions. For this reason, the packaging step becomes cumbersome, and the manufacturing efficiency of the semiconductor device degraded.
Therefore, a circuit board having high-strength characteristics resistant to external force, high-toughness or tenacity characteristics, high output, and excellent heat radiation characteristics coping with high heat generation is demanded.
In a circuit board formed such that a metal circuit layer and heat-generating parts such as semiconductor elements are integrally bonded to each other on the surface of an aluminum nitride substrate, the mechanical strength and toughness of the aluminum nitride substrate itself are insufficient. For this reason, the circuit board receives a repetitive heat cycle, and cracks are easily formed in the aluminum nitride substrate near the bonding portion of the metal circuit layer. Therefore, heat cycle resistance characteristics and reliability are degraded.
In a case wherein a circuit board is manufactured by using a ceramic substrate such as aluminum nitride having high thermal conductivity, in order to assure a certain strength value and high dielectric strength, an aluminum nitride substrate having a large thickness must be used. For this reason, regardless of the high thermal conductivity of the A1N substrate, the heat resistance value of the overall circuit board increases. Therefore, heat radiation which is in proportion to the thermal conductivity cannot be obtained.
The present invention has been made to cope with the above demands, and has as its object to provide a silicon nitride circuit board which uses high-strength and high-toughness characteristics which are original characteristics of a silicon nitride sintered body, and which has a high thermal conductivity, excellent heat radiation characteristics, and considerably improved heat cycle resistance characteristics.
It is another object of the present invention to provide a silicon nitride circuit board which has a high thermal conductivity, excellent heat radiation characteristics, and considerably improved heat cycle resistance characteristics, and in which assembling or packaging characteristics to a semiconductor device in the assembly step are improved.